Powder coating compositions are extremely desirable for use in painting substrates in that they are essentially free of organic solvents conventionally utilized in liquid paint systems. Thus, they give off little, if any, volatile material to the environment when heat cured.
Powder coatings comprising (1) an epoxy-functional and hydroxy-functional copolymer, (2) a monomeric anhydride or a homopolymer of a monomeric anhydride, and (3) a hydroxy carboxylic acid have been described in U.S. patent application Ser. No. 394,876, filed Sept. 6, 1973 by A. N. Theodore, a coinventor herein, E. C. Siwiec and H. Van Oene. Powder coating compositions comprising (1) an epoxy-functional copolymer and (2) an anhydride cross-linking agent have been described heretofore in our co-pending U.S. patent application Ser. No. 172,224, filed Aug. 16, 1971, now U.S. Pat. No. 3,781,379; in U.S. Pat No. 3,758,632; in U.S. patent application Ser. No. 394,887, filed Sept. 6, 1973 by A. N. Theodore, a coinventor herein, E. C. Siwiec and H. Van Oene, in U.S. patent application Ser. No. 394,878, filed Sept. 6, 1973 by A. N. Theodore, a coinventor herein, E. C. Siwiec and H. Van Oene, in our U.S. patent application Ser. No. 394,879, filed Sept. 6, 1973; in U.S. patent application Ser. No. 394,880, filed Sept. 6, 1973 by S. S. Labana, a coinventor herein, and S. C. Peng and in our U.S. patent application Ser. No. 394,881, filed Sept. 6, 1973. Powder coating compositions comprising (1) an epoxy-functional, hydroxy-functional copolymer and (2) a dicarboxylic acid cross-linking agent are disclosed in our U.S. patent application Ser. No. 394,874 filed Sept. 6, 1973. Powder coating compositions comprising (1) an epoxy-functional, amide-functional copolymer and (2) a carboxy terminated polymer, e.g., a carboxy terminated polyester, are disclosed in U.S. Pat. application Ser. No. 394,875 filed Sept. 6, 1973 by J. D. Nordstrom and S. C. Peng.
In the preparation, storage and use of powder coating compositions a variety of problems arise which are either peculiar to powder coating materials or are common to other forms of coating but differ in degree or form in powder coating materials. These include lack of component compatibility, i.e., phase separation, resistance to pigment dispersion, caking, etc. For example, both monomeric anhydrides and homopolymers of monomeric anhydrides, e.g., poly (azelaic anhydride) and poly (adipic anhydride), make effective crosslinking agents for suitable copolymers, e.g., epoxy-functional copolymers. Care must be exercised, however, in the preparation and use of such coating compositions in that monomeric anhydrides tend to sublime during processing and powders containing polyanhydrides of the type above described have a tendency to cake.